As is known, inherent in the conventional carbureted multicylinder automobile engine are various factors which influence the driveability of the automobile. One such principal factor is the distribution of the air/fuel mixture to the various cylinders through the intake manifold.
In the past any problems arising from the distribution of the fuel in the induction system of the automobile engine has been mitigated or solved by operating the engine on a richer air/fuel mixture, one having a greater than stoichiometric mixture for complete combustion. This mixture was generally chosen to obtain maximum power with maximum fuel economy.
In order to comply with legislative controls for automotive emission, automotive manufacturers now design automobile engines to operate on leaner air/fuel ratios approaching the stoichiometric mixture, that is, one containing the chemically correct proportions of fuel and air for complete combustion. Although operating with these leaner ratios helps control hydrocarbon and carbon monoxide emissions, the cylinder-to-cylinder distribution problem is accentuated and it is no longer possible to solve the distribution problem by using the richer air/fuel mixtures. Though this lean carburetion is a principal factor, it is not the only cause for poor driveability.
Additionally, the engine warm-up period immediately subsequent to cold start is very difficult to control and in the past choke control was longer in order to improve driveability during the warm-up period. However, in order to cut back on exhaust emissions, longer choke periods are no longer permissible and the resulting leaner air/fuel mixture used during the warm-up cycle has also accentuated the driveability problem.
The partially-oxidized metal layers comprising the carburetor and intake manifold interior surfaces have high critical surface tension and wettability and are completely wet by any hydrocarbon gasoline component, either aliphatic or aromatic. The mixture of gasoline and air that leaves the carburetor and passes to the various cylinders through the intake manifold thereby tends to deposit some of the higher boiling fractions particularly rich in aromatics in the form of a liquid film on these surfaces particularly on the walls of the intake manifold. For the best distribution of the fuel in the induction system, the gasoline should be present as a vapor in the air/fuel mixture; therefore, this wetting of this surface contributes to less satisfactory distribution and poor driveability.
It has been disclosed by A. A. Zimmerman et al, "Improved Fuel Distribution -- A New Role for Gasoline Additives," Paper 720082 presented at Automotive Engineering Congress, Detroit, January, 1972 (SAE Transactions, Sec. I, Vol. 81 (1972), pp. 316-324) that low energy surfaces can be produced in the intake manifold by inclusion of additives in the gasoline at concentrations of less than 100 ppm which deposit out gradually, or by precoating of the manifold surfaces. Zimmerman et al recommend materials with critical surface tensions less than about 22 dynes/cm. The value of this physical property is determined from a plot of surface tension of a homologous family of alkanes (hexane, heptane, octane, decane) against the cosine of their contact angles, .theta., on a surface; all alkanes having surface tensions equal to or lower than the value of the critical surface tension of the particular surface completely wet the surface. Thus the teaching of Zimmerman et al is relevant for alkanes or the aliphatic portion of gasoline. All contemporary gasolines contain an aromatic fraction. At the intake manifold a great majority of the liquid volume of gasoline, mainly the light alkanes, has been flashed off leaving the heavier aromatics as the major constituent of the liquid drops on the intake manifold surface. Thus, it has now been found that there is an increased criticality of aromatics and the wettability thereof at the intake manifold surface in contrast to the aromatics in gasoline in the fuel tank of the automobile or in the carburetor. It has now been discovered that the essence of supplying an advantageous driveability-enhancing additive or coating, especially a fluorine-containing polymer, for the intake manifold surface is to select one that has a high contact angle with toluene, a representative aromatic liquid, thereby obtaining intake manifold surfaces that repel aromatics, i.e., they have low wettability by aromatics. While it appears that a high contact angle or low critical surface tension with alkanes is of some significance in attaining good driveability characteristics with gasolines, it is now apparent that the real critical factor in obtaining enhanced driveability is this low wettability of the manifold surface by aromatics, a factor which heretofore has neither been acknowledged or even recognized in the prior art, e.g., see the cited paper by Zimmerman et al.
While Zimmerman et al point out the disadvantages of using fluorocarbon additives to coat the intake manifold surfaces in terms of cost and possibility of release of toxic and corrosive hydrogen fluoride in the exhaust, they indicate that coating the intake manifold surface with a fluorine-containing polymer such as Teflon.sup.R tetrafluoroethylene polymer, which has a critical surface tension of 18 dynes/cm and repels alkanes having surface tensions greater than this value, produces partial wetting of the surface by gasoline and improves the distribution and the driveability. However, in order to achieve these effects, the intake manifold surface must be mechanically coated with the Teflon.sup.R polymer; this means the intake manifold surface must be precoated with the Teflon.sup.R polymer, for example, during manufacture of the engine, or, for cars already on the road, it would be necessary to rebuild the engine. In U.S. Pat. No. 3,791,066, issued February 12, 1974, it is proposed to avoid the use of polymers by applying low-molecular weight fluorine-containing materials which are highly soluble in gasoline. These materials are fluorinated aliphatic hydrocarbon, fluorinated aliphatic hydrocarbyl amine, or fluorinated aliphatic hydrocarbyl amine salt, that is, hydrocarbyl or amino-substituted hydrocarbyl fluorides.